1. Field of the Invention
The present invention concerns a system for measuring the transfer time of a sound-wave, in particular for continuously measuring the transfer time of a sound-wave through a gas which is in a state of turbulence at a high temperature, and in such cases as involve a corrosive environment, with the ultimate goal of continuously measuring the temperature. The invented system is also suited to measuring other associated parameters of the said transfer time within a body, such as its relative velocity or distance from a determined reference point.
2. Description of the Prior Art
For the measurement of the temperature of a gas, the state of the art includes the use of conventional pyrometers such as thermocouples and thermoresistors but also more advanced systems which utilize the existing relation between the velocity of sound propagation through a gas and the temperature of this gas: an emitter is stimulated by a suitable electric signal and generates an acoustic vibration in the gas, while a microphone suitably placed relative to the emitter receives the said vibrations which are then transformed into electrical signals.
The transmitted signals and received signals are compared by appropriate algorithms to provide the so called "time of flight", or transfer time that is the time which the signal emitted by the emitter takes to arrive at the microphone. From this transfer time one is able to ascertain the average temperature of the gas passed through by the acoustic vibration.
The more advanced of the systems are of two different types: those which use a commercial type "spark-gap" device to generate the sound with a single pulse, at high voltage and high current, and those which use a siren, such as those used at sports stadiums to generate the sound. With the first type of system a packet of waves without a well defined frequency or phase is emitted, and one attempts to correlate the time of arrival of the envelope of received signals with the instant of emission of the electrical pulse excited by the "spark-gap", while with the second type of system a wave train of variable and well repeatable frequency is emitted, but with intrinsic incoherence of phase, and a real and proper "cross correlation" between the wave shape of the emitted signal and the wave shape of the received signal is performed.
The disadvantages of conventional pyrometers (thermocouples, thermoresistors, etc) are found in applications where one wants to measure an average temperature, in the necessity of using many of them and in subsequently taking the average, and in the difficulty of their use in hot and very corrosive environments. The disadvantages of pyrometers of an acoustic type for the measurement of the transfer time lie principally in the uncertainty of an exact determination of the transfer time at the very moment one is measuring the temperature of a gas in the presence of raised noise levels produced by the turbulence of the gas, by a burner, by structural vibrations and the like (in the case of the "spark-gap" the uncertainty derives from the fact that the emitted signal is a non-repeatable signal and so its decoding is affected by measurement statistic, while in the second case the cross-correlation between the emitted and the received signals is not very efficient in the presence of the noise and echoes associated with turbulence); in the most favorable of conditions one can obtain a measurement of the accuracy of the estimate of the transfer time in the order of the median period of the emitted wave, which is unsatisfactory in the majority of cases (for example, with f=2000 Hz, the median period and therefore the precision of the measurement is about .+-.0.5 milliseconds and this gives uncertainties in the order of .+-.100.degree. C. in paths of about 10 m in an environment at a temperature of about 1000.degree. C.).